3 pencils cost $5.31. Which equation would help determine the cost of 15 pencils?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. If we let $x$ represent the cost of 15 pencils, we have the following proportion: $\dfrac{x}{15}$ We have to pay $5.31 for 3 pencils, and that can be written as a proportion: $\dfrac{\$5.31}{3}$ Since the price per pencil stays the same, these two proportions are equivalent. $\dfrac{x}{15} = \dfrac{\$5.31}{3}$